Frequency modulation



May 3, 1949. J. G. BEARD ETAL FREQUENCY MODULATION 2 Sheets-Sheet 1 Filed Feb. 26, 1945 INVENTORS. c/Zui/w 554mg.

' '4 Nam 5K May 3, 1949. J. G. BEARD ETAL 2,469,289

FREQUENCY MODULATION Filed Feb. 26, 1945 2 Sheets-Sheet 2 I N VEN TORS.

J Jays/w 6. 55420.

160 W 50PM BY V Patented May 3, 1949 UNITED STAT ES. PATENT OFFICE FREQUENCY MODULATION Joseph G. Beard, Haddonfield, and Lee W. Born,.

Collingswood, N. J;, assignors to Radio Corp! ration ofAme-ricma corporation of Delaware- Application February 26, 1945, Serial No. 579,802

6' Claims.

1': This application relates to frequency modulation systems and in particular to means for'gencrating oscillatory energy of carrier wave frequency the frequency of which may be shifted as desired from band'to band, means for modulating the frequency ofthe generated oscillatory energythrough a desired range orsweep, and means for changing" the-range-or sweep when the band in which the oscillator is operating is changed tohcld the-percentage modulation as desired. In a preierred embcdiment the oscillaticns are modulated bytonefrequency and themodulat-ion range is so controlled thatthe amount of modulation is constant;

Tone modulated generators of the type de- 2 and 3, each illustrate-the essential features of a wave generator andmodulator and percentage modulation regulating means arranged in accordance with my invention, while Fig. 1a illustrates details'of anelementofthe arrangement of Fig. 1.

In Fig. 1, l0 designatesgenerally a tube oscillation generator which. may be of any type and for'the sake of 'simplicity'is' shown as a triode tube with its input and output electrodes regeneratively coupled by a tank circuit l2- and a feedback circuit I l. The tank :circuit' I2 may include an inductance and a bank of condensers It, a selected one of which may be connected by a; switch Si in shunt to the inductance of tank circuit 12. condenser flll'may beconnected in shunt to this circuit. InductancelZ-isaiso shunted by an additional condenser formedby=plates Pl andP2; one of which is substantially fixedand the other of which, sayPZ, is movable andzis connected to the tank circuit l2'so that the capacity between. P! and PZxshunts this circuit, The plate P2 is ectcd to'thevibratory'element 22.01" a me- I cal vibrator comprising as'is'well known in t electrical art, magnetic windings: 24

mounted asshown'on field poles and-connected with a source 28,. through :an element 28: against which the vibratory: element 22 is retained'by- Ifdesired, an additional variable spring 39. The elements,- 22' to 3!! are known in the art as a buzzer, and are proportioned and adjusted to vibrate at the desired" modulation whichmay be a tonefrequency. As the member 22vibrates the distance between the electrodes PI and PZis varied to thereby vary' frequency frequency the capacity in the tank circuit l2 and modulate the generated oscillations. Whenthe unmodulated operating frequency of the generator I0 is changed bychanging switch S1 a metallic masking disc 32- 'which' extends between the plates PI and PZ'iis rotated on'its axis 33 to Thiscoupling is varied due to' the' fact that as shownin Fig. 1a the masking'd isc 32- has therein an opening the sizeof'whi'ch changes as the disc is vary the coupling between'Pland P2.

rotated on its axis. In the embodiment. illustrated when the-switch'Sl is changed' to put a. larger condenser of the bank 16 in the tank cir-'- cuit the metallic disc 32 is simultaneously rotated?- to present a larger. opening between the plates P and P2 because more capacitywar-iation is required at the low frequencyuend'of the range than at the high frequency'end; By properly:

, proportioning the opening inthemasking or shield plate 32 the amount of frequency shift of the oscillator lll'maybe held constant over theentire range thereof;

In the embodiment.illustrated'inFi'g; 2, theplate 32 is not used.

quency ban-d ofoperation of the'oscillatorasse lected by switch S1; Switch S2,: which may be a section of switch Sl,

vibration of plate Pz holds the amount of modulation constant. The-small series. capacitors of bank 35 mayv be selected by the same switch which selects the desiredfrequencyband or may be selected by a separate switch S2. These ca-.

pacitors may be made-adjustable to thereby permit compensation fortheinherent tolerances and variations in the components of I the oscillator circuit including tank. circuit l2.

In this embodiment the: bank of con'densers'and %switch S1 operate as in the prior embodiment; as-does the capacity beoperates in unison with switch Si and the bankof condensers 36 is so. arranged. that the effective capacity variation'or In the embodiment illustrated in Fig. 3, the mechanical vibrator now comprises a tuning fork 40 operated by a fork driver 42 in a well known manner to oscillate or vibrate at a frequency depending upon its dimensions. Located adjacent to one tine 44 of the fork 40 is a series of adjustable electrodes 46 each of which forms the plate of a separate condenser, the other plates of which are formed by the said one tine of the fork 40. These condensers are adjustable by movement of the screws on which they are mounted. The relative movement of the fork tine with respect to the adjustable electrodes 46 provides the required capacity variation for the desired frequency modulation of the oscillator. The condensers formed by the plates 46 and tine 44 are connected in shunt to the tank circuit I2 of the oscillator Ill so that as the tine vibrates the generated oscillations are modulated in frequency. Since the tipof the tine 46 sustains greater amplitude of vibration, the resultant capacity change will be larger for the electrode 46 placed at the tip of the fork and the resultant capacity variation will decrease as the electrode is moved along the fork, assuming the same are equally or substantially equally placed from the tine. Thus the electrode at the tip of the tine is selected by the oscillator frequency band switch S2 for the lowest frequency band, as are electrodes 46 toward the base of the fork for successively higher hands. This produces an inherent variation in the magnitude of the vibrating capacitance which is such as required to maintain a uniform frequency shift with frequency band change. Each electrode 46 is individually adjustable so that the precise adjustment may be obtained for each frequency band. The range of operation is as in the prior embodiment changed by changing the switch SI or a section thereof to select a desired condenser of the bank l6 and connect the same in shunt to the tank circuit I2, and at the same time couple a selected condenser plate 46 in shunt to the tank circuit [2.

In the embodiments described, the amount of modulation is to be held constant irrespective of the frequency at which the generator operates in the unmodulated condition. It will be understood that in accordance with our invention the reactances fixing the extent of swing may be proportioned so as to hold the percentage modulation constant over the operating range of the oscillator or other relations between the amount of swing and the carrier frequency may be established as desired by proper proportioning of P1, P2, 32 and the condensers of bank 36.

What is claimed is:

1. In a frequency modulated oscillation generator, an electron discharge device having its electrodes coupled in wave generating circuits including reactances at least two of which are variable, one of said reactances comprising condensers of different value, switching means for including a selected one of said condensers in said circuits to change the frequency at which said generator operates, said other of said reactances comprising a variable condenser having a fixed plate and a movable plate, mechanical means for varying said movable plate at a tone rate to modulate the frequency of the oscillations generated, and mechanical means linked to said switching means for operation therewith for changing the effective capacity of said variable condenser when the frequency of operation is changed to hold the extent of modulation deviation of the oscillations generated throughout the range of operation of said generator substantially constant.

2. A modulated generator as recited in claim 1 wherein said first mechanical means is a mechanical vibrator operating at tone frequency and linked to said movable plate.

3 A frequency modulated oscillator as recited in claim 1 wherein said second mechanical means for changing the effective capacity of said variable condenser is a metallic disc-like element between the plates of said variable condenser and having an opening therein the size of which varies with movement, and wherein said disc-like element is moved by being linked to said switching means, said disc-like element acting as an electrostatic shield between the plates of the variable condenser.

4. In a timing modulated wave generator, an electron discharge device having its electrodes coupled in wave generating circuits including a variable reactance and a bank of substantially fixed reactances, a switch for selecting one of said reactances of said bank of reactances to change the frequency at which said generator operates, mechanical means for varying the other of said reactances at a tone rate to modulate the timing of the oscillations generated, and mechanical means linked to said switch for varying the range through which said other reactance is varied when the fixed reactance is changed by I said switch to maintain substantially constant the extent of modulation of the generated wave throughout the range of operation of said generator.

5. In a frequency modulated wave generator, an electron discharge device having its electrodes coupled in wave generating circuits including reactances at least two of which are variable, means for varying one of said reactances to change the frequency band in which said generator operates, the other of said variable reactances comprising a selected one of a plurality of capacitors, one plate of each of which is fixed and spaced along the length of a tuning fork 1 which forms the other plates of said capacitors to provide a plurality of capacitors the capacity variations of which depend on the said spacing along the length of the tuning fork, means for vibrating said tuning fork by varying said selected one of the plurality of capacitors at a predetermined rate to modulate the frequency of the generated wave, and means, linked with said first named means, for changing said other reactance included in said generator circuit, when said one reactance is varied, to maintain substantially constant the amount of modulation of said generated wave energy throughout the range of operation of the generator.

6. In a frequency modulated oscillation gen erator, an electron discharge device having its electrodes coupled in wave generating circuits including reactances at least two of which are variable, one of said reactances comprising condensers of different value, switching means for including a selected one of said condensers in said wave generating circuits to change the frequency at which said generator operates, said other of said reactances comprising a variable capacitor arrangement including fixed and movable plates, mechanical means for varying said movable plates at a modulation rate to modulate the frequency of the oscillations generated, and mechanical means linked to said switching means for operation therewith for changing the effective capacity of said variable capacitor arrangement 5 6 when the frequency of operation is changed to hold the extent of modulation deviation of the UNITED STATES PATENTS oscillations generated throughout the range Of Number Name Date operation of said generator substantially con- 1,897,767 Roberts Feb. 14, 1933 stant. 5 1,972,964 Wolcott Sept. 11, 1934 JOSEPH G. BEARD. 2,075,125 Mabry Mar. 30, 1937 LEO W, BORN, 2,205,190 Farrington June 18, 1940 2,301,869 Hammond Nov. 10, 1942 REFERENCES CITED 2,339,608 Alvira Jan. 18, 1944 2,355,338 Stewart Aug. 8, 1944 The following references are of record in the 10 file of this patent: 

